What's the meaning of the phrase 'Mackerel sky'?
A Mackerel sky is a sky that is streaked with rows of small white clouds which resemble the pattern of scales on a mackerel's back.
What's the origin of the phrase 'Mackerel sky'?
There was no idiomatic dexterity involved in the derivation of this phrase; mackerel skies do look like the markings on a mackerel's back.
The term has been in use since the 17th century and was first out into print by the appropriately named Thomas Sprat, in The History of the Royal-Society of London, for the improving of natural knowledge, 1667:
Let Water'd signifie a Sky that has many high thin and small Clouds, looking almost like water'd Tabby, called in some places a Mackeril Sky.
Meteorologically speaking, a mackerel sky is created when mid-level moisture is trapped between dry air below and cold dry above.
The compressing of the cloud between the two air pockets forms the characteristic rippled formation.